test-bdrv-drain: Call bdrv_co_unref() in coroutine context

bdrv_unref() is a no_coroutine_fn, so calling it from coroutine context
is invalid. Use bdrv_co_unref() instead.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230510203601.418015-7-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
1 file changed